How can I achieve a domed or "hangar" type cross-section shape?
Shift+A Select Add a Mesh then select Add a Circle from the drop down menu. You can change the number of vertices in the tool bar on the left.

Press Tab to go into Edit Mode. Select the number of vertices desired to delete bye selecting them and then pressing X and then vertices from the drop down menu.

Select the bottom two vertices and press F to connect them.

Select all of the vertices and press E and then press the letter of the axis you want to extrude them along. For example press E and then Y to extrude along the Y axis.